Instructions: Preheat oven to 350 degrees. Grease a 9-by-13 inch baking pan.

Combine butter, shortening, brown sugar, egg and vanilla in a medium bowl.

Beat at medium speed of electric mixer until well blended.

Combine graham cracker crumbs, flour and salt. Add to creamed mixture. Mix at low speed until just blended. Spread dough evenly into prepared pan.

Bake in preheated oven 15 minutes, or until edges are lightly browned and set. (The center of the crust will still be a little soft.) Cool completely.

Place cream in a heavy saucepan and bring to a simmer. Remove from heat. Add chocolate pieces. Stir until melted and smooth. Stir in marshmallows. Spread evenly over cooked crust. Refrigerate until firm, about 2 hours, before cutting into bars.

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